1. Field of the Invention
The present invention relates to an optical system useful as a photographic optical system of an image pickup apparatus, such as a silver-halide film camera, a digital still camera, a video camera, a digital video camera, or a TV camera.
2. Description of the Related Art
It has become desirable that a photographic optical system used in a single-lens reflex digital camera and a single-lens reflex film camera has a wide angle of view and a long back focus. As a photographic optical system having a wide angle of view and a long back focus, U.S. Pat. Nos. 5,559,638 and 5,315,441 each discuss a retrofocus type photographic optical system.
In the retrofocus type photographic optical system discussed in each of U.S. Pat. Nos. 5,559,638 and 5,315,441, a lens unit having a negative refractive power (a first lens unit) is provided in a front portion thereof (in a photographic optical system such as a camera, on an object side). In a rear portion of the above-described photographic optical system (in a photographic optical system such as a camera, on an image side), a lens unit having a positive refractive power is provided. The above-described configuration discussed in each of U.S. Pat. Nos. 5,559,638 and 5,315,441 implements a photographic optical system having a wide angle of view and a long back focus.
Japanese Patent Application Laid-Open No. 2002-303790 discusses a retrofocus type photographic optical system whose first lens unit having a negative refractive power includes a plurality of negative lenses and a lens having an aspheric shape to effectively correct off-axis aberration. U.S. Pat. No. 5,805,349 discusses a small size photographic optical system having a long back focus configured by increasing the refractive power of a negative lens of a first lens unit having a negative refractive power, which is located closest to the object side.
Generally, in a retrofocus type photographic optical system, a lens unit having a negative refractive power is provided in front of an aperture stop, and a lens unit having a positive refractive power is provided behind the aperture stop. Therefore, a retrofocus type photographic optical system is asymmetric as a whole.
In addition, in a retrofocus type photographic optical system, in order to achieve a long back focus, an absolute value of the negative refractive power of the front lens unit (the first lens unit) is high. Accordingly, in the retrofocus type photographic optical system described above, a great amount of aberrations may occur.
In particular, as the angle of view becomes wider, the more asymmetric the arrangement of refractive powers may become. More specifically, a large amount of aberrations, such as coma, astigmatism, and spherical aberration, may occur. Furthermore, in a retrofocus type photographic optical system described above, a large amount of off-axis aberrations that have occurred in a first lens unit having a negative refractive power may remain uncorrected. In particular, a large amount of distortion and chromatic aberration of magnification may remain uncorrected.
Accordingly, in a retrofocus type photographic optical system, it is significant to appropriately set the configuration of lenses of the front lens unit (the first lens unit) in order to achieve a photographic optical system having a wide angle of view and a long back focus and capable of capturing a high quality image on the entire image plane.